In the art, expanded bed absorption (EBA) type chromatographic systems are an example of chromatographic separations where a filter bed can be movable, for example in a transition wherein the filter bed moves from a compressed to an expanded state. Indeed, where classical column chromatography uses a solid phase made by a packed bed, EBA typically uses a fluidized bed. Particles such as whole cells or cell debris, which would clog a packed bed column, readily pass through a fluidized bed. EBA can therefore be used on crude culture broths or slurries of broken cells. The feed flow rate is kept relatively low so that the solid packing remains stratified and does not fluidize completely. In the known expanded absorption type chromatographic systems the filtered substance is extracted by a top collector connected to the column's effluent port. However, in a situation where the filter bed is varied the quality of the filtered substance may be low, since the top collector will have a varying distance relative to the upper stratum of the filter bed, which may cause unwanted back mixing to take place. A chromatographic system of the above-identified type is described in U.S. Pat. No. 4,249,828